1. Field of the Invention
The present invention relates generally to modulating the growth, differentiation and/or cell division of meristematic cells in a plant, more particularly modulating said cells via modulating RRB gene activity.
2. Description of Related Art
The ability to control the size of plants and plant tissues is an enormously valuable tool. For example, for many agricultural crops, increasing the size of a plant or of a specific tissues within a plant would be of obvious commercial value. Currently, most attempts to increase plant size or yield are accomplished through traditional or marker-assisted breeding programs. Such methods have, however, failed to provide methods to directly control the size of plants or plant tissues.
Most cell proliferation in plants occurs in tissues called meristematic tissue. Several types of meristematic tissue exist in plants, including the shoot apical meristem, which gives rise to all aerial parts of the plant, the root apical meristem, which establishes the root system, and the vascular meristem, which provides lateral growth of the plant. While several genes are known to alter meristem fate, and thereby plant development, the mechanism by which they function is poorly understood. The products of the CLAVATA (CLV) and SHOOT MERISTEMLESS (STM1) genes of Arabidopsis, for example, encoding a receptor-kinase and homeodomain protein, respectively, appear to work antagonistically in a shoot meristem maintenance pathway involved in the partitioning of the central-peripheral zone (CZ-PZ) of the meristem. Other genes, such as ZWILLE (ZLL) and WUSCHEL (WUS), function early in embryonic development to specify the stem cells which will be maintained in the central zone of the shoot apical meristem. Other genes such as MGOUN1(MGO1) and MGOUN2 (MG02) appear to function in the partitioning of cells from the PZ of the shoot apical meristem to leaf primordia or the inflorescence, often resulting in a fasciated meristem phenotype.
Plants containing mutations in the genes described above are defective in specific stages of meristem function and have well-characterized developmental phenotypes. As such, these genes are likely involved in the differentiation of meristematic cells, and are thus unlikely, by themselves, to provide tools to increase the size of plants or of plant tissues. Instead, it would be desirable to manipulate both the differentiation of meristematic cells as well as their growth and proliferation.
One potential method to alter the growth and/or proliferation of plant cells would be to modulate the activity of genes controlling these processes. For example, several groups have reported the cloning of at least a fragment of a Retinoblastoma-related protein in maize. See, e.g. Ach et al. (1997) Mol. Cell. Biol. 17:5077; Huntley et al. (1998) Plant Mol. Biol. 37:155; Grafi et al. (1996) PNAS 93:8962; Shen et al. (1994) Plant Mol. Biol. 26:1085; Xie et al. (1996) EMBO J 15:4900; and WO 97/47745. None of these studies, however, has investigated the function of RRB in proliferating, virus-free cells. Further, no studies have heretofore addressed the role of RRB in an intact plant. As well known to those of skill, only by examining the role of a protein in its normal environment, in an intact organism, can its true activity and/or function be determined.
Thus, the art lacks a good understanding of the function of RRB in plant cells and/or intact plants. Without this understanding, its use to control plant growth in an efficient manner is difficult if not impossible. The present invention addresses these and other needs.